Field of the Invention
The invention relates to a battery-powered electrical device comprising a rechargeable battery pack and a switchable voltage drain, the battery pack containing a set S of individual cells in series connection, the set S having a cardinal number N greater than two, each cell being characterized by:
a nominal voltage V.sub.bat ; PA1 a minimal permissible voltage-value V.sub.min ; PA1 a voltage-value V.sub.end at which it is desirable to terminate discharge of the cell. PA1 the set S is divided into a number of mutually exclusive subsets s.sub.i of cardinal number n.sub.i, whereby 0&lt;n.sub.i &lt;N, at least one cardinal number n.sub.i is greater than 1, and the sum of all the cardinal numbers n.sub.i is equal to N; PA1 n.sub.i .ltoreq.n, where n is the largest natural number satisfying the relationship: PA1 the device comprises means for measuring the series voltage V.sub.i across each subset s.sub.i, and interrupting the switchable voltage drain if the condition: PA1 is met in any of the subsets s.sub.i. PA1 (1) The stipulation: PA1 can alternatively be written in the form: PA1 (2) If, in a given subset s.sub.i, the weakest cell has discharged as far as the minimum permissible voltage V.sub.min, then the maximum possible voltage across the subset s.sub.i can never be more than: PA1 (3) Combining this with the previous inequality, it transpires that: PA1 However, this falls within the cut-off condition: PA1 It thus transpires that discharging cannot continue beyond the point where one of the cells has discharged as far as the minimum permissible voltage.
The term "cardinal number" refers to the number of elements in a set, i.e. in this case the number of cells in the set S. The term "nominal voltage" refers to the (rated) voltage across the terminals of the cell when it is loaded ("battery pole voltage"), and specifically refers to the average voltage on the plateau of the cell's discharge curve. The value of V.sub.min for a given application is a selected voltage-level through which even the weakest cell in the battery pack should not be allowed to discharge, under any circumstances. The value of V.sub.end for a given application is a selected voltage-level below which further discharge of the cell is of little use, since the remaining cell voltage will be insufficient to satisfactorily operate the device.
2. Description of Related Art
Devices of this type are well known from everyday experience, and include, for example, electric power tools, video cameras, mini vacuum cleaners, lap-top computers, electronic notebooks, portable music sources, torches, mobile communication devices, etc. Because such devices are commonly designed to operate at a rated voltage which is higher than the nominal voltage V.sub.bat of common cells (e.g. 1.25 V), the devices must accordingly derive their power from series-arrays of such cells (i.e. battery packs).
Many commonly-available battery-powered devices contain a voltage drain (e.g. a motor, lamp, electrical circuit, etc.) which simply continues to draw power from the battery pack until the pack's residual voltage is no longer sufficient to operate the device; the operator is then expected to recharge the battery pack before further use. However, it is now generally known that such depletory usage can be detrimental to the performance of the battery pack in the long run, since discharge of cells beyond a certain point can cause the occurrence of irreversible chemical reactions within the cells, which can damage their performance. For this reason, it is generally desirable to interrupt power to the voltage drain well before the battery pack becomes depleted, and then to recharge the battery pack before further use.
One way of performing this interruption is to choose a voltage-value V.sub.end at which it is desirable to terminate discharge of the cell, to continually measure the voltage V.sub.pack across the pack, and to interrupt power to the voltage drain as soon as V.sub.pack =N.times.V.sub.end (cut-off monitoring). A problem with such an approach, however, is that, if the pack contains a weak cell, then such a cell can still be over-discharged, despite the above-mentioned cut-off procedure. For example, if N=5, V.sub.bat =1.25 V, V.sub.min =0 V and V.sub.end =0.9 V, then N.times.V.sub.end =4.5 V. However, if one of the cells is empty and the other four are full, then, initially, V.sub.pack =4.times.1.25 V+0 V=5 V, which is greater than N.times.V.sub.end (=4.5 V), so that cut-off will not yet occur. Under these circumstances, discharge of the battery pack will continue, despite the fact that one of the cells in the pack is already completely discharged. This will generally result in irreversible damage to the weak cell, causing it to weaken even further.
An alternative is to individually monitor each cell, and to interrupt the voltage drain as soon as the voltage across any given cell reaches V.sub.end. However, this approach requires considerable extra circuitry, especially in the case of battery packs with relatively large values of N (more than 5, for example).